1. Field of the Invention
The present invention relates to a terahertz wave generation element for generating terahertz waves including electromagnetic wave components in a frequency domain from a millimeter wave band to a terahertz wave band (30 GHz to 30 THz), a terahertz wave detection element for detecting a terahertz wave, and a terahertz time domain spectral device using at least one of the above elements. In particular, the present invention relates to a generation element including an electro-optic crystal for generating or detecting an electromagnetic wave including a Fourier component in the above frequency domain by irradiating laser light and a tomography device using a terahertz time domain spectral method (THz-TDS) including the generation element.
2. Description of the Related Art
In recent years, a nondestructive sensing technique using a terahertz wave has been developed. As an application field of electromagnetic waves in this frequency band, there is a technique for forming a safe examination device, which is an alternative to an x-ray device, and performing imaging. Also, a spectroscopic technique for studying physical properties such as a molecular bonding state by obtaining an absorbing spectrum and a complex dielectric constant in a material, a measuring technique for studying physical properties such as carrier concentration, carrier mobility, and electric conductivity, and an analysis technique for biomolecules have been developed. As a method for generating a terahertz wave, a method using a nonlinear optical crystal is widely known. Typical examples of the nonlinear optical crystal include LiNbOx, (hereinafter also referred to as LN), LiTaOx, NbTaOx, KTP, DAST, ZnTe, GaSe, GaP, and CdTe. A second-order nonlinear phenomenon is used to generate a terahertz wave. As a method, difference-frequency generation (DFG) by inputting two laser beams having frequencies different from each other is known. Also, monochromatic terahertz wave generation by an optical parametric process and a method for generating terahertz pulses by optical rectification by irradiating a femtosecond pulse laser beam are known.
As a process for generating a terahertz wave from such a nonlinear optical crystal, electro-optic Cerenkov radiation attracts attention recently. This is a phenomenon as shown in FIG. 7 in which, when a propagation group velocity of laser beam 100 which is an excitation source is faster than a propagation phase velocity of a generated terahertz wave, a terahertz wave 101 is emitted conically like a shock wave. A radiation angle θc is determined by the following formula using a ratio of refractive index of light to a terahertz wave in a medium (nonlinear optical crystal).cos θc=vTHz/vg=ng/nTHz 
Here, vg and ng respectively represent a group velocity and a group refractive index of excitation light, and vTHz and nTHz respectively represent a phase velocity and a refractive index of the terahertz wave. Regarding the Cerenkov radiation phenomenon, there is a report in which a monochromatic terahertz wave is generated by the DFG method using a slab waveguide having a thickness sufficiently smaller than a wave length of the generated terahertz wave (“Opt. Express, vol. 17, pp. 6676-6681, 2009”: Document 1).
Such an example of Document 1 is generation of a terahertz wave by traveling wave excitation, so this relates to a proposal in which terahertz waves generated from different wave sources are phase-matched in radiation direction and strengthen each other and thereby extraction efficiency is improved. The features of the radiation method are that relatively high efficiency of radiation can be achieved and a high intensity terahertz wave can be generated when a nonlinear optical crystal is used, and the frequency band of the terahertz wave can be widened by selecting absorption of terahertz area by phonon resonance peculiar to a crystal on the high frequency side. In these techniques, the frequency band can be wider than that of terahertz waves generated by a photoconductive device, and the pulse width can be narrowed in a case of terahertz pulse generation using optical rectification, and for example, it is expected to improve device performance when these techniques are used in a terahertz time domain spectral device.
However, in the method described in Document 1, electric field intensity distribution in a cross-section of a radiated terahertz wave beam is asymmetrical with respect to the optical axis. This is because the wavelength of the light is dispersed as the light propagates in a waveguide and the electric field intensity of the generated terahertz wave decreases.